1. Field of the Invention
This invention relates to a digital magnetic recording/reproduction apparatus and method for handling digital information signals and, more particularly, to a magnetic recording/reproduction system with a calibration system for widening a window margin used for data discrimination.
2. Description of the Related Art
Compact data processing apparatuses such as personal computers and word processors generally use a floppy disk drive (FDD) as an external memory device. It is desired that the floppy disk for use in such data processing apparatuses have a greater memory capacity. There are two measure to increase the memory capacity of the floppy disk. One is to increase the track density of the recording medium, the other being to increase the medium's bit density. In the FDD, to increase the track density, the FDD will inevitably encounter a limit in the improvement of a mechanical accuracy of the recording medium and a positioning accuracy of the magnetic head. In this respect, the increase of the bit density is desired. In the magnetic recording/reproduction apparatus, such as the FDD, handling the digital information signal when the bit density of the floppy disk is increased, a peak shift of the information signal reproduced from the disk increases, so that a window margin for data discrimination decreases. When the reproduced digital information signal is discriminated by the window signal, the peak shift possibly results in a deviation of the data, contained in the reproduced signal, from the window signal on the time axis. If this data deviation -- which is known as a bit deviation, -- occurs, this gives rise to a data discrimination error. The window margin means the time margin defined by a difference between the data deviation and the pulse width of the window signal. If the window margin is narrow, the data discrimination error may easily occur. The window margin varies depending not only on the peak shift, but also a spacing between the recording medium and the magnetic head, recording current, the characteristic of the recording medium, and saturation of the magnetic head.
Turning now to FIG. 7, this figure shows a characteristic curve representing the relationship between a dipulse ratio (B/A) and the thickness of a protective layer of the recording medium. This curve was plotted by recording and reproducing an isolated wave signal by use of a digital magnetic recording/reproduction apparatus incorporating a perpendicular magnetic recording medium and a ring type magnetic head. As can be seen from FIG. 7, the dipulse ratio represents a ratio of the amplitude (B) of a negative pulse in the reproduced isolated wave signal to that (A) of the positive pulse. The perpendicular magnetic recording medium was a floppy disk structured so as to have a Co-Cr thin film, covered with a protective layer, formed on a flexible substrate. The diameter of the flexible substrate was 3.5 inches, the Co-Cr film had a perpendicular magnetic anisotropy, being 970 Oe in coercivity (Hc) and 440 G in saturation magnetization (Ms), and the surface of the protective layer was lubricated. The magnetic head was an Mn-Zn ferrite head having a gap length of 0.31 .mu.m. In FIG. 7, the dipulse ratio was measured in varying the thickness of the protective layer. The characteristic curve shown in FIG. 8 represents a relationship between a recording current fed to the magnetic head and the dipulse ratio of the reproduced signal. In the case of FIG. 8, the dipulse ratio was measured by varying the recording current, while the thickness of the protective layer is fixed to 200 .ANG..
As can be seen from FIGS. 7 and 8, when the thickness of the protective layer, which is equal to an effective spacing between the recording medium and the magnetic head, and the recording current increases, the dipulse ratio of the reproduced signal waveform steeply decreases. As a result, a longitudinal waveform component contained in the reproduced signal waveform having the longitudinal component and a perpendicular components, is increased. The increase in the thickness of the protective layer and the increase of the recording current cause the recording magnetization mode to change from a perpendicular mode to a longitudinal mode. As a result of this change in mode, the waveform of the reproduced signal also changes.
The recording magnetization mode also changes depending on the spacing between the head and the recording medium which changes, a change in the recording current due to characteristics of the recording media used, or the difference in recording magnetic field intensity due to the difference in characteristic between the magnetic heads used. When the reproduced signal waveform changes in accordance with the recording magnetization mode, the peak shift changes, and hence the window margin changes. Therefore, the change of the recording magnetization mode gives but an insufficient window margin, and possibly causes a data discrimination error.
In an apparatus using a longitudinal magnetic recording medium, increase of the recording current changes the pulse width of the reproduced signal waveform. Also in this case, the window margin reduces, and the same problem occurs.
In actual use of the FDD, it is frequently required to secure a compatibility of the upper-grade device with the lower-grade device. For example, when the upper-grade device is designed to use the perpendicular magnetic recording medium, while the lower-grade device is designed to use the longitudinal magnetic recording medium, it is desirable that the higher-grade device handle both types of the recording media. The conventional recording/reproduction apparatus can handle, only the perpendicular recording medium or the longitudinal recording medium. If one tries to use for a magnetic recording/reproduction apparatus a recording medium different from that originally used, the window margin is remarkably decreased, rendering the use of the recording medium impractical.